A two-level inverter topology, as illustrated in FIG. 1, is commonly implemented using either all silicon (Si) technology or all silicon carbide (SiC) technology. That is, the switching transistors are implemented in Si technology using Si IGBTs (Insulated Gate Bipolar Transistors) with Si anti-parallel diodes. Alternatively, the switching transistors are SiC MOSFETS (Metal Oxide Semiconductor Field Effect Transistors) and the anti-parallel diodes are SiC Schottky diodes. Currently, the two-level inverter topology based on Si IGBTs and Si diodes is widely used and remains an established industry standard solution for automotive applications, such as vehicle traction inverters for electric vehicles (EV) and hybrid electric vehicles (HEV). Other applications include, for example, photovoltaic grid inverters, and PFC (Power Factor Correction) rectifiers, motor controllers, and power supplies. Si IGBTs can block high voltages, have low on-state conduction losses, and well-controlled switching times. A two-level converter based on Si IGBTs provides low conduction losses, a small part count and simple operation, at low cost. Thus, there are continued efforts to provide improved performance of lower cost, two-level inverters using Si IGBTs.
SiC MOSFETS and diodes offer performance advantages for the two-level inverter topology, but at significantly higher cost. For example, an article by P. Kierstead, entitled “Inverter design optimized using all-SiC power devices”, 30 Jan. 2013, (www.electronicsprodurts.com) provides a comparison of two-level inverter designs using SiC power devices and Si IGBTs. A presentation entitled “Power Electronics for Electric Vehicles”, STMicroelectronics, APEC 2017, provides a comparison of Si versus SiC technologies for a two-level converter topology, e.g., a 80 kW EV traction inverter.
An example of a three-level T-type Neutral Point Clamped (NPC) inverter topology is shown in FIG. 2. The conventional two-level topology is extended with a neutral clamping leg comprising an active, bidirectional switch to the DC-midpoint. The operation of a three-level T-type Neutral Point Clamped (NPC) converter of this topology, based on Si IGBT components is described in detail in an article by M. Schweizer et al., entitled “Design and Implementation of a Highly Efficient Three-Level T-type Converter for Low-Voltage Applications”, IEEE Transactions on Power Electronics, Vol. 28, No. 2, February 2013. This 3-level T-type NPC inverter topology is reported to provide reduced switching losses and superior output voltage quality relative to a conventional 2-level inverter topology.
An article by E. Avci et al., entitled “Analysis and design of a grid-connected 3-phase 3-level AT-NPC inverter for low voltage applications” Turk. J. Elec. Eng. & Comp. Sci. (2017) 25: 2464-2478 (doi:10.3906/31k-1603-159), v. 29 May 2017, discloses an all silicon solution using Si IGBTs for a T-Type 3-level NPC inverter in which the middle bidirectional switch of the neutral clamping leg uses Reverse Blocking IGBTs (RB-IGBTs) that provide both forward and reverse blocking capabilities, which are reported to be more efficient because switching and conduction losses are reduced.
Application note AN-11001, entitled “3L NPC and TNPC Topology”, SEMIKRON International GmbH, 12 Oct. 2015 (www.semikron.com), provides a detailed review of the operation of 3-level diode clamped NPC inverter topologies and 3-level T-type NPC inverter topologies implemented with all Si IGBTs and anti-parallel FWDs, for applications in the range of 800V to 1500V, and from 5 kW to 250 kW.
An article by M. Ikonen, et al., entitled “Two-Level and Three-Level Converter Comparison in Wind Power Application” (2005) provides an analysis of power losses in a 2-level topology and 3-level diode clamped topology, 3-level flying capacitor inverter topology, and cascaded H-bridge inverter topology, using Si IGBT technology.
Wide bandgap (WBG) semiconductor technologies, such as SiC and GaN technologies offer performance advantages, such as, higher efficiency, higher switching frequencies, and reduced losses. An article by R. Allan, entitled “SiC and GaN vs. IGBTs: The Imminent Tug of War for Supremacy” in Power Electronics, 27 Jul. 2017, provides a brief overview of the benefits of GaN HEMTs and SiC MOSFETs vs. Si IGBTs. With respect to EV and HEV traction inverters, a presentation entitled “Gallium Nitride Power Transistors in the EV World,” GaN Systems Inc., June 2017, discloses an example of a 48V 12 kW 2-level HEV traction inverter using GaN HEMTs, which provides improved efficiency, reduced losses, and higher power density, in an air-cooled module which is one fifth the size and one third the weight of a comparable Si MOSFET inverter. On the other hand, use of Si IGBT technology for power applications is well-established and offers reliable performance at low cost. Also, the latter presentation notes that power applications span a wide range of voltages from low voltage, e.g. 100V to 300V for consumer electronics and power supplies; medium voltage, e.g. 650V to 1200V for applications such as EV/HEV traction inverters, PV inverters, motor controllers and UPS; and high voltage above 1700V, for applications such as smart power grid, wind power generation, and large-scale transport, e.g. rail and shipping. Thus, it is likely that each of Si, SiC and GaN technologies will continue to co-exist and offer complementary solutions for different automotive, industrial, consumer and other power applications.
For further background information on implementation of 3-level T-type NPC inverter topologies using all SiC technology and using all GaN technology, reference is made, by way of example, to the following recently published articles.
An article by A. Anthon et al., entitled “The Benefits of SiC MOSFETs in a T-Type Inverter for Grid-Tie Applications”, IEEE Transaction on Power Electronics, Vol 32, No. 4, April 2017 (doi: 10.1109/TPEL.2016.2582344) v. 20 Jan. 2017, provides a comparison of all Si and all SiC implementations of T-Type NPC 3-level inverters.
An article by H. Kurumatani et al., entitled “GaN-HEMT-Based Three Level T-type NPC Inverter Using Reverse-Conducting Mode in Rectifying” presents an all GaN solution for a 3-level T-type NPC Inverter using 100V/4 A GaN HEMTs for low voltage applications;
An article by R. Chen et al., entitled “Design and Implementation of a Three-Phase Active T-Type NPC Inverter for Low Voltage Microgrids”, Energy and Power Engineering, 2017, 9, pp 70-77, Apr. 6, 2017 (DOI 10.4236/epe.2017.94B009) discloses an all GaN solution using GaN HEMTs for 3 kW three-phase inverter for low-voltage micro-grids, operable over switching frequencies from 3 kHz to 60 kHz.
Another all GaN implementation is disclosed by M. Ferdowsi et al., in a presentation entitled “Gallium Nitride (GaN) based High Frequency Inverter for Energy Storage Applications”, EESAT 2017 Conf. Proceedings, 11 Oct. 2017. This all GaN solution uses 650V GaN HEMTs in an alternative active-clamped 3-level NPC inverter topology.
A few hybrid implementations of 3-level T-type NPC converters have been proposed. For example, Japanese patent publication no. JP2011078296 A, 14 Apr. 2011, entitled “Power Conversion Circuit”, by Azuma Satoshi discloses a hybrid implementation of a 3-level T-type NPC power converter topology using Si IGBTs and anti-parallel diodes for all switches, wherein the Si anti-parallel diodes of the neutral clamping leg are replaced with wide bandgap (WBG) diodes. United States patent no. US2015/0108958 23 Apr. 2015, J. Xu et al., entitled, “Hybrid Three-level T-type Converter for Power Applications” discloses a hybrid implementation wherein the outer switching transistors are wide bandgap transistors, e.g. SiC JFETs or SiC MOSFETs, and the transistors of the neutral clamping leg are Si MOSFETS or GaN HEMTs. The article by Anthon et al., referenced above, proposes a hybrid 3-level T-type NPC inverter topology wherein the outer switching transistors are SiC MOSFETS, and switches of the neutral clamping leg are Si IGBTs with anti-parallel diodes.
In view of the reliability and relatively low cost of Si IGBTs and diodes, and their widespread and established use for power applications, there is an ongoing demand for Si IGBT based inverters and rectifiers with enhanced performance for applications, such as, EV and HEV traction inverters, photovoltaic grid inverters and motor controllers.